The invention concerns a power steering system of the type which includes a steering servo-motor with a steering assist piston acted on by fluid pressure in respective oppositely facing working chambers and a servo-valve for controlling fluid pressure supply to the working chambers in dependance on the position of a servo-valve adjusting member which is operatively connected to the vehicle hand steering wheel. Reaction pressure chambers at opposite sides of the servo-valve adjusting member are connected by pressure connections to the working chambers to communicate steering feedback to the driver. This type of system includes modulating valve means for modulating the reaction pressure which is communicated back to the steering servo-motor, typically as a function of engine speed.
When parking with a power steering system of this type--i.e. with a high requirement for steering assistance and low travelling speed--large quantities of leakage oil can reach the pressure medium return via the modulating valve while by-passing the steering system servo-motor.
An object of the invention is to so arrange a power steering system of this type that the modulating valve can be in a shut-off position, in which flow through it is prevented, when the travelling speed tends toward zero.
According to one aspect of the invention a modulating valve is arranged in a short-circuit bypass connection between the two reaction chambers and the pressure connections between the reaction chambers and working chambers are continuously open.
In the power steering system according to certain preferred embodiments of the invention, the two reaction pressure chambers of the control valve are short-circuited via the throttle even when the modulating valve is in the shut-off position, so that, on the one hand, quantities of leakage oil are avoided and, on the other hand, the function of the control valve is not adversely affected.
In a known power steering system (German Pat. Specification No. 1,040,392), the modulating valve is inserted between the reaction pressure chamber and the valve connection, associated with the operating pressure chamber, of the control valve, so that two pressure connections are necessary between the modulating valve and the control valve and the unreduced operating pressure of the power steering system arises as the output pressure at the modulating valve during a corresponding steering activity. In this known power steering system, the constructional and spatial requirement for the control valve is also large because the reaction pressure chamber has to be arranged fully independent of the valve connections of the control valve.
In a known power steering system of another type (German Offenlegungsschrift No. 2,851,773), a modulating valve operating as a function of the travelling speed and/or other parameters is, indeed, used to produce a reaction pressure dependent on the parameters but the feedback which can be felt at the hand steering wheel results from a differential pressure brought into effect at the valve setting element by means of additional reaction pressure pistons. This differential pressure is produced from the full operating pressure and the reaction pressure set by the modulating valve, so that the differential pressure which can be felt as the feedback can assume pressure values between zero and the actual value of the operating pressure. In this known power steering system, the arrangement of reaction pressure chamber and piston associated with steering to the right and the arrangement associated with steering to the left are synchronously subjected to pressure, so that special means are necessary to deal with the pressure force of the arrangement not associated with the steering direction being used.
In especially preferred embodiments of the invention, the modulating valve is arranged to shut off the pressure connection at low traveling speeds.
In especially preferred embodiments of the invention, the respective pressure connections which bypass the modulating valve and connect the reaction and operating pressure chambers are provided with a constant throttle and the modulating valve is configured to produce a pressure drop in the reaction pressure chamber at least indirectly relative to a pressure medium return which is substantially relieved of pressure.
In particularly preferred embodiments some of the reaction pressure chambers are located so as to be fixed in position relative to the valve housing of the servo-control valve. Preferred embodiments further include a bypass throttle connection containing a throttle and connection parallel with the modulating valve. In especially preferred embodiments there is provided a valve for limiting the pressure-dependent feedback connected in parallel with the modulating valve.
In certain preferred embodiments a controllable shut-off valve arrangement is connected in parallel with the throttles disposed in the lines connecting the respective reaction and working chambers, which shut-off valve arrangement is located between the modulating valve and the pressure medium return.
In certain preferred embodiments, the reaction pressure chamber is connected to the respective operating pressure chamber via the associated valve connection of the servo-control valve.
Preferred embodiments of the present invention exhibit the following advantages:
(i) only one pressure connection is necessary between the modulating valve and the reaction pressure chamber and a reduced output pressure relative to the operating pressure arises at the modulating valve by means of the throttle;
(ii) it is possible to operate with lower pressure values and flow rates for the reaction pressure due to the throttle inserted between the modulating valve and the pressure medium return;
(iii) no extra reaction pistons are necessary for producing the reaction pressure force;
(iv) the characteristics of the feedback can be modified by a bypass throttle connection, containing a throttle, to the modulating valve;
(v) the feedback can be limited by a valve connected in parallel with the modulating valve;
(vi) the characteristics of the feedback can be modulated by a controllable shut-off valve arrangement, which is connected in parallel with the throttle located between the modulating valve and the pressure medium return;
(vii) the reaction pressure chamber is connected to the operating pressure chamber via the associated valve connection of the control valve;
(viii) the reaction pressure chamber is connected to the associated valve connection of the control valve by means of a pressure duct, which contains the throttle, of the valve setting element;
(ix) the two reaction pressure chambers, each connected on one side to an operating pressure chamber, are connected on the other side to one another by means of a short-circuit pressure connection containing a common modulating valve;
(x) the shut-off valve arrangement is inserted in a pressure duct of the valve setting element of the control valve;
(xi) the throttle, located between the modulating valve and the pressure medium return, is arranged downstream of the valve, for limiting the feedback;
(xii) the valve for limiting the feedback is inserted directly between the reaction pressure chamber and the pressure medium return; and
(xiii) the modulating valve is inserted in series with at least one throttle between the operating pressure source, on one side, and a pressure medium return, substantially relieved of pressure, during a steering deflection of the manual steering wheel, the two reaction pressure chambers, each connected on one side to an operating pressure chamber, being connected to one another on the other side by means of a short-circuit pressure connection.
In order to be able to avoid (particularly when parking--i.e. at low travelling speeds) a flow through the modulating valve leading to undesirable leakage quantities, it is also advantageous in the power steering system according to the invention that the short-circuit pressure connection contains the throttle located in series with the modulating valve and the modulating valve is inserted in the pressure connection between one reaction pressure chamber and the associated operating pressure chamber. In this manner, the modulating valve can be closed at low travelling speeds without the control valve being blocked in the process.
Further objects, features, and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.